Method and installation for the production of hot-rolled strip having a dual-phase structure

ABSTRACT

The aim of the invention is to be able to produce dual-phase steels under local conditions even in the existing cooling section of a continuous casting and rolling plant by means of controlled cooling of the hot-rolled strip in two cooling stages following the forming process. Said aim is achieved by respecting the chemical composition of the initial steel within precisely defined limits and cooling in two stages from a finished rolled strip temperature T finish  of A 3 −100 K&lt;T finish &lt;A 3 −50 K to a coiling strip temperature T coiling  of &lt;300° C. (&lt;initial martensite temperature). the cooling speed V 1,2  in both cooling stages ranging between 30 and 150 K/s, preferably between 50 and 90 K/s. The first cooling stage is carried out until the cooling curve enters the ferrite range, whereupon the heat released by the transformation of the austenite into ferrite is used for isothermally holding the obtained strip temperature T const  during a holding time of =5 s until the beginning of the second cooling stage.

The invention concerns a method for producing hot-rolled strip with adual-phase microstructure consisting of ferrite and martensite, whereinat least 70% of the austenite is transformed to ferrite from thehot-rolled state by a controlled two-stage cooling operation after thefinish rolling to a strip temperature below the martensite starttemperature in a cooling line that consists of successive, spaced watercooling units.

Systematic microstructural transformation by controlled cooling issteels is well known, and to produce dual-phase steels, this controlledcooling is carried out after the working of the hot strip is complete.The adjustment of the attainable dual-phase microstructure dependsessentially on the cooling rates that are technically possible in theinstallation and on the chemical composition of the steel. In thisregard, it is important in any case to achieve sufficient ferriteformation of at least 70% in the first cooling stage. During this firstcooling stage, transformation of the austenite in the pearlite stageshould be avoided.

The cooling capacity of the second cooling stage following the firstcooling stage must be sufficiently high that coiling temperatures belowthe martensite start temperature are reached. Only then is the formationof a dual-phase microstructure with ferritic and martensiticconstituents ensured.

The previously known production of dual-phase steels is unproblematic atlow strip speeds or with sufficiently long cooling lines. However, atvery high strip speeds, the beginning of the second cooling stage can beshifted so far in the present cooling line that the subsequentmartensite formation remains incomplete or does not occur at all. Thisresults in a mixed microstructure consisting of ferrite, bainite andsome martensite, so that the desired mechanical properties of a puredual-phase microstructure are not obtained.

EP 0 747 495 B1 describes a method for producing high-strength steelplate with a microstructure consisting of 75% ferrite, at least 10%martensite and possibly bainite and retained austenite. Accordingly,this is not a microstructure of pure dual-phase steels. A steelmicroalloyed with niobium is used as the alloy. It is produced bysystematically cooling the hot-rolled steel plate, wherein a rapidcooling follows a slow cooling or, alternatively, a rapid coolingprecedes the slow cooling. A cooling rate of 2-15° C./s within a coolingtime of 8-40 s is given for the first cooling stage to a finaltemperature between the AR, point and 730° C. In the second coolingstage, the steel is cooled to a temperature of 300° C. at a cooling rateof 20-150° C./s. In the alternative method, in which the rapid coolingstage precedes the slow cooling stage, rapid cooling is carried out to atemperature below the Ar₃ point at a cooling rate of 20-150° C./s.

EP 1 108 072 B1 describes a method for producing dual-phase steels, inwhich a dual-phase microstructure consisting of 70-90% ferrite and30-10% martensite is achieved with a two-stage cooling operation (firstslow, then rapid) carried out after the finish rolling. The first (slow)cooling is carried out in a cooling line in which the hot-rolled stripis cooled in a well-defined way by successive, spaced water coolingzones at a cooling rate of 20-30 K/s. In this connection, the cooling isadjusted in such a way that the cooling curve enters the ferrite rangeat a temperature that is still so high that ferrite formation can occurrapidly. The first cooling is continued until at least 70% of theaustenite has transformed to ferrite. This cooling stage is immediatelyfollowed by the other (rapid) cooling stage without any holding time.

Proceeding from the aforementioned prior art with the various possiblemeans that have been described for producing dual-phase microstructure,the objective of the invention is to specify a method by which and aninstallation in which the production of hot-rolled strip with dual-phasemicrostructure can be carried out in a conventional continuous castingand rolling installation with the local limitations that exist there andthus with the given time constraints. The cooling line of aninstallation of this type is characterized by the fact that the totallength generally does not exceed 50 m and that compact cooling is notprovided.

The objective with respect to the method is achieved with thecharacterizing features of claim 1. The method is characterized by thefact that, to obtain a hot-rolled strip with a dual-phase microstructureconsisting of 70-95% ferrite and 30-5% martensite with high mechanicalstrength and high formability (tensile strength greater than 600 MPa,elongation after fracture at least 25%) in the cooling line of acontinuous casting and rolling installation, starting from a steel withthe following chemical composition: 0.01-0.08% C, 0.9% Si, 0.5-1.6% Mn,1.2% Al, 0.3-1.2% Cr, with the remainder consisting of Fe and customarytrace elements, the two-stage controlled cooling is carried out from afinish rolling strip temperature T_(finish), such that A₃=100K<T_(finish)<A₃=50 K, to a coiling strip temperature T_(coiling)<300° C.(<martensite start temperature), wherein the cooling rate V_(1,2) inboth cooling stages is V=30-150 K/s, and preferably V=50-90 K/s, thefirst cooling stage is carried out until the cooling curve enters theferrite range, and then the heat of transformation liberated by thetransformation of the austenite to ferrite is used for isothermallyholding the strip temperature thereby reached for a holding time of 5 suntil the beginning of the second cooling stage.

Due to the short length of conventional cooling lines in existingcontinuous casting and rolling installation, the production ofhot-rolled strip with a dual-phase microstructure is possible only witha special cooling strategy. To allow a special cooling strategy of thistype to be carried out, it is absolutely necessary to maintain certainlimits of chemical composition, such as those listed in claim 1, so thatthe desired degree of transformation can be achieved with the shorttotal cooling time that is available.

The cooling strategy involves two cooling stages that have selectivelyvariable cooling rates and are interrupted by an isothermal holding timeof a maximum of 5 s. The beginning of the holding time, whichcorresponds to the end of the first cooling stage, is determined by theentrance of the cooling curve into the ferrite range, i.e., the point atwhich the austenite starts to transform to ferrite. The entire desiredtransformation of the austenite to at least 70% ferrite occurs in theshort isothermal cooling interruption of a maximum of 5 s, during which,in accordance with the invention, the liberated heat of transformationholds the temperature at a constant value by compensating unavoidableair cooling. This holding time is then immediately followed by thesecond cooling stage, during which the hot-rolled strip is cooled to atemperature below 300° C. Since this temperature is below the martensitestart temperature, the desired level of martensite, which is the secondconstituent of the dual-phase microstructure, is thus obtained.

In addition to the use of a short holding time, the cooling strategy isdefined by an exactly defined, predetermined cooling rate for bothcooling stages. This cooling rate is V=30-150 K/s, and preferablyV=50-90 K/s. It depends on the geometry of the hot-rolled strip and onthe chemical composition of the grade of steel that is used. In regardto these cooling rates, it should be noted that a cooling rate of lessthan 30 K/s is not possible due to the small amount of time available inthe conventional cooling line of a continuous casting and rollinginstallation, and cooling rates greater than 150 K/s also cannot beattained in conventional cooling lines.

Compared to prior-art methods for producing dual-phase hot-rolled strip,the method of the invention is characterized not only by the fact thatthe initial steel has a different chemical composition but also by thefact that

(a) the finish rolling temperature is well below the A₃ temperature,

(b) cooling is carried out to a temperature below 300° C. in the secondcooling stage,

(c) the cooling rates are below 150 K/s and above 30 K/s,

(d) there is a very short holding time of a maximum of 5 seconds, duringwhich no cooling occurs, between the two cooling stages, and

(e) the transformation to ferrite occurs isothermally.

A continuous casting and rolling installation for carrying out themethod of the invention is characterized by a conventional cooling linethat is installed after the last finishing stand and has severalsuccessive, spaced water cooling units, which can be automaticallycontrolled. The spray bars present in each cooling unit are arranged insuch a way that a specific amount of water is uniformly sprayed onto theupper and lower surfaces of the hot-rolled strip. The total amount ofwater can be automatically controlled by turning individual spray barson or off during rolling. The number and arrangement of the water spraybars that are turned on can be variably adjusted in advance to obtain anoptimum adjustment of the entire cooling line to the cooling conditionsthat are to be established.

Further details, features and characteristic of the invention areexplained in greater detail below with reference to the specificembodiment of the invention that is illustrated in the schematicdrawings.

FIG. 1 shows a time-temperature cooling curve of a hot-rolled strip.

FIG. 2 shows a layout of a cooling line in a continuous casting androlling installation with a 6-stand finishing train.

FIG. 3 shows a layout of a cooling line in a continuous casting androlling installation with a 7-stand finishing train.

FIG. 1 shows an example of a time-temperature cooling curve of ahot-rolled strip that was cooled by the method of the invention on therunout roller table in a cooling line 1. The hot-rolled strip, which hadthe following composition: 0.06% C, 0.1% Si, 1.2% Mn, 0.015% P, 0.06% S,0.036% Al, 0.15% Cu, 0.054% Ni, 0.71% Cr, the remainder consisting of Feand customary trace elements, was cooled in a first cooling stage at acooling rate V₁ of 54 K/s from an adjusted finish rolling temperatureT_(finish) of 800° C. to a hot-rolled strip temperature of 670° C., atwhich the cooling curve entered the ferrite range. During a holding timeof about 4 s, the temperature of the hot-rolled strip remained at thisholding temperature T_(const.), and then the final cooling was carriedout in a second cooling stage, in which the strip was cooled to atemperature below 300° C. (about 250° C. coiling temperature) at acooling rate V₂ of 84 K/s. Tests on the hot-rolled strip produced bythis method, which had a dual-phase microstructure in the desired rangeof at least 70% ferrite and less than 20% martensite, yielded a tensilestrength of 620 MPa combined with a ratio of yield stress to tensilestrength of 0.52.

FIG. 2 shows an example of a layout of a cooling line 1 of aconventional continuous casting and rolling installation. The coolingline 1, through which the hot-rolled strip passes in direction ofconveyance 8, is located between the last finishing stand 2 and thecoiler 5. A temperature-measuring point 6 for monitoring the temperatureof the hot-rolled strip 10 entering the cooling line 1 is locatedbetween the last finishing stand 2 and the first water cooling unit 3 ₁.The cooling line 1 shown in FIG. 2 comprises a total of eight coolingunits 3 ₁₋₇ and 4. The latter is often realized as a trimming zone 4.More generally, a conventional cooling line comprises six to ninecooling units, depending on the particular continuous casting androlling installation.

The example illustrated in FIG. 2 is the typical layout of a coolingline for a 6-stand continuous casting and rolling installation, as isapparent from the gap between cooling units 3 ₇ and 4. Subsequentconversion to a 7-stand finishing train often requires that, forexample, the first cooling unit (cooling zone) 3 ₁ be moved to the rearinto the structural gap between the cooling units 3 ₇ and 4. In thiscase, the cooling line 1′ has a layout of the type shown in FIG. 3,which differs from the layout of the cooling line 1 in FIG. 2 only bythe elimination of this structural gap between the cooling units 3 ₇ and4. Therefore, the reference numbers of the individual structural partsand assemblies of FIG. 3 are the same as the reference numbers of FIG.2. An exception to this is the first cooling unit 3 ₁′, whose upperspray bar, in contrast to the spray bar of cooling unit 3 ₁ in FIG. 2,is designed with the standard length of the cooling units 3 ₂ to 3 ₇.

In most cases, each cooling unit has four spray bars on both the upperside and the lower side. Each spray bar in turn consists of two rows ofsmall water pipes for cooling the upper surface of the strip 10′ and thelower surface of the strip 10″. As a special feature, the cooling unit 3₁ in FIG. 2 is shortened by one spray bar on the upper side due tolimited space.

In contrast to the upstream cooling units 3 ₁₋₇, which have oneswitchable valve 7 per spray bar, the trimming zone 4 has two valves 7for each spray bar. This means that in the trimming zone, each row ofsmall cooling pipes can be individually controlled, and thus the amountof water can be more finely controlled.

The delivery speed of the strip from the finishing train varies with therolled thickness of the finished strip. Accordingly, the mode ofoperation of the cooling line must be adjusted to be able to adjust thetime-temperature control necessary for the adjustment of the stripproperties. For a strip thickness of 3 mm, for example, the firstrequired cooling level is attained with the cooling units 3 ₁ and 3 ₂,while the second cooling level is realized with cooling units 3 ₅, 3 ₆,3 ₇, and 4. Due to the altered boundary conditions for a finished stripwith a thickness of 2.0 mm, only cooling units 3 ₆, 3 ₇, and 4 need tobe used for the second cooling stage.

LIST OF REFERENCE NUMBERS

-   1 cooling line-   2 last finishing stand-   3 ₁₋₇ water cooling units-   4 water cooling unit (trimming zone)-   5 coiler-   6 temperature-measuring point-   7 switchable valve-   8 direction of conveyance-   10 hot-rolled strip-   10′ upper surface of the strip-   10″ lower surface of the strip-   V₁ cooling rate of the first cooling stage-   V₂ cooling rate of the second cooling stage-   T_(finish) strip temperature after the last finishing stand-   T_(const.) strip temperature after the holding time-   T_(coiling) strip temperature at the end of cooling (coil    temperature)

1. Method for producing hot-rolled strip (10) with a dual-phasemicrostructure consisting of ferrite and martensite, wherein at least70% of the austenite is transformed to ferrite from the hot-rolled stateby a controlled two-stage cooling operation after the finish rolling toa strip temperature below the martensite start temperature in a coolingline (1, 1′) that consists of successive, spaced water cooling units (3₁₋₇, 4), wherein, to obtain a hot-rolled strip (10) with a dual-phasemicrostructure consisting of 70-95% ferrite and 30-5% martensite withhigh mechanical strength and high formability (tensile strength greaterthan 600 MPa, elongation after fracture at least 25%) in the coolingline of a continuous casting and rolling installation, starting from asteel with the following chemical composition: 0.01-0.08% C, 0.9% Si,0.5-1.6% Mn, 1.2% Al, 0.3-1.2% Cr, remainder Fe and customary traceelements: (a) the two-stage controlled cooling is carried out from afinish rolling strip temperature T_(finish), such that A₃=100K<T_(finish)<A₃=50 K, to a coiling strip temperature T_(coiling)<300° C.(<martensite start temperature), wherein the cooling rate V_(1,2) inboth cooling stages is V=30-150 K/s, and preferably V=50-90 K/s, and (b)the first cooling stage is carried out until the cooling curve entersthe ferrite range, and then the heat of transformation liberated by thetransformation of the austenite to ferrite is used for isothermallyholding the strip temperature thereby reached T_(const.) for a holdingtime of 5 s until the beginning of the second cooling stage. 2.Continuous casting and rolling installation for producing hot-rolledstrip (10) with a dual-phase microstructure from the hot-rolled state,with a cooling line (1, 1′), which is installed after the last finishingstand (2) and has several successive, spaced water cooling units (3 ₁₋₇,4), for carrying out the method in accordance with claim 1, wherein thecooling line (1, 1′) has a standard length (<50 m) for conventionalcontinuous casting and rolling installations, within which a suitablenumber of automatically controllable water cooling units (3 ₁₋₇, 4) arearranged in such a way that the required cooling rate (V_(1,2)) of eachcooling stage can be adjusted and the required holding time at the striptemperature T_(const.) between the two cooling stages can be realized byan adapted mode of operation of the entire cooling line as a function ofthe strip thickness and the strip speed.
 3. Continuous casting androlling installation in accordance with claim 2, wherein each watercooling unit (3 ₁₋₇, 4) contains several spray bars that can beautomatically controlled by switchable valves (7), that the spray barsare arranged in such a way that the upper surface (10′) and the lowersurface (10″) of the hot-rolled strip (10) passing through the coolingline are uniformly sprayed with a certain amount of water, and that theamounts of water for the upper surface (10′) and the lower surface (10″)of the strip can be trimmed even relative to each other.
 4. Continuouscasting and rolling installation in accordance with claim 3, wherein thelast water cooling unit (4) for cooling the upper surface (10′) and thelower surface (10″) of the strip has eight switchable valves (7) foreach four spray bars on the top and on the bottom to allow more exactadjustment of the amount of water.